Vibration and condition monitoring system and the parts thereof

ABSTRACT

A vibration and condition monitoring system, with true digital signal processing based design, with very limited analog based general signal conditioning and integrated specific sensor conditioning in each module. In addition to the support for common eddy current probe systems, employing an external driver, the module also supports direct connection of an eddy current probe system to the module, due to a built-in driver and linearization functionality. Specific sensor signal conditioning is not dependent on hardware, but only on embedded software, firmware. There is full sensor input support in an I. S. environment. Not only the common sensor input types from accelerometer, velocity sensor, but also direct input for eddy current probe systems for both vibration and/or speed measurements. The module also comprises means to assess the type and correct functionality of an attached eddy current probe system ( 302, 303 ) by means of a frequency measurement and possibly an amplitude measurement.

TECHNICAL FIELD

The invention concerns eddy current probe driver systems and relateddigital linearization either as a part of a digital processing module oras a stand-alone eddy current probe driver module, especially inhazardous zones. The invention also concerns vibration and conditionmonitoring systems.

BACKGROUND

Eddy Current Probe (ECP) sensor systems have been used since the 1970'sfor non-contact displacement measurements, in the monitoring andprotection of rotating machines mainly operating with journal (sleeve)bearings. ECP systems are also commonly known as “Proximity ProbeSystems”.

Eddy current probes are dependent on a driver comprising an oscillator.The oscillator is used to excite the attached eddy current probe so thatit can generate a changing magnetic field. This first magnetic field,when in close range to a steel target material, will induce timechanging eddy currents in the surface of the target material. These eddycurrents, in turn, will generate a second magnetic field that willoppose the originating first magnetic field and therefore affecting theresulting impedance of the probe tip. The size of the induced eddycurrents is dependent on the distance between the probe tip and thesteel target material. The probe impedance change is therefore a directmeasurement of the distance between the probe tip and the targetmaterial.

Oscillator circuits in eddy current driver systems are commonly basedupon a Collpits type oscillator using discrete matched transistor stagesas active elements. The oscillator stage is current driven and basicallyoperated in fully saturated mode, acting as a switch and thus providingthe required energy to sustain an oscillation. The resulting amplitudeis defined by the non-linearity of the drive currents and is temperatureand device dependent as a result of parasitic influences. In order toattain a similar output response from multiple modules, this very lowcost method requires amplitude calibration and also temperature andfrequency compensation due to the used PN junctions of the drivingtransistors. In addition, amplitude stability is dependent on thestability of the load of the oscillator circuit due to output impedanceand parasitic capacitors. Besides variations in parasitic influencesbetween components, these capacitive load influences will also befrequency dependent and therefore affect the overall probe/cable tankimpedance value.

It is desirable that the oscillation frequency remains as constant aspossible over the full operation range of a probe system even withvarious cable lengths. Furthermore, in order to tune the tank circuitfor optimum impedance response for longer cable lengths, a parallel,commonly ferrite-based, load inductance is usually used in currentsystems. This ferrite-based inductor, however, will experience long-termchange/drift over time and thus also have an effect on the tank circuitoutput impedance.

By using a constant drive current, the output voltage is a function oftank impedance and fundamental harmonic response of the excitationcurrent. Due to these characteristics, for equal probe/cable systems,the resulting tank voltage is subject to level changes, and thereforecannot be assumed to be constant between different driver modules. Thistherefore requires undesirable manual calibration, and calls for bettermeans of amplitude accuracy and stability. It is further a desire toverify the functionality of an attached probe/cable system for whichthere seems to be room for improvement.

SUMMARY

An object of the invention is to define a method of driving and a driverof eddy current probes.

Another object of the invention is to define a method of driving and adriver of eddy current probes that enables functionality and/or typeverification of an attached probe/cable system.

Still another object of the invention is to define a method of verifyingthe functionality and/or type of a probe/cable system attached to aneddy current driver system.

A further object of the invention is to define a unit and a method ofenabling efficient vibration monitoring in hazardous areas.

The aforementioned objects are achieved according to the invention by amethod of determining a status of an eddy current probe/cable systemattached to an eddy current probe oscillator According to the inventionthe determination of the status is based on a frequency of the eddycurrent probe oscillator. Suitably determining the status according tothe method comprises determining a system type of the eddy currentprobe/cable system and/or determining a correct functioning of the eddycurrent probe/cable system. The method can preferably comprise aplurality of steps. In a first step a frequency of the eddy currentprobe oscillator is measured. In a second step the measured frequency iscompared with one or more previously measured frequencies, and/or apredefined frequency, and/or a predefined range of frequencies. In athird step the status is determined by means of the result of thefrequency comparison. The method can suitably further comprisesadditional steps. In a first additional step the frequency of the eddycurrent probe oscillator is demodulated. In a second additional step anamplitude of the demodulated frequency is measured. In a thirdadditional step the measured amplitude is compared with one or morepreviously measured amplitudes, and/or a predefined amplitude, and/or apredefined range of amplitudes. And also in that the third step ofdetermining the status also comprises determining the status by means ofthe result of the amplitude comparison.

The aforementioned objects are achieved according to the invention by aunit arranged to determine a status of an eddy current probe/cablesystem attached to an eddy current probe oscillator. According to theinvention the determination of the status is based on a frequency of theeddy current probe oscillator. Suitably determining the status comprisesdetermining a system type of the eddy current probe/cable system and/ordetermining a correct functioning of the eddy current probe/cablesystem. Preferably the unit comprises measurement means, frequencycomparison means and determining means. The measurement means isarranged to measure a frequency of the eddy current probe oscillator.The frequency comparison means is arranged to compare the measuredfrequency with a one or more previously measured frequencies, and/or apredefined frequency, and/or a predefined range of frequencies. Thedetermining means is arranged to determine the status by means of theresult of the frequency comparison means. Suitably the unit furthercomprises demodulation means, measurement means, and amplitudecomparison means. The demodulation means is arranged to demodulate thefrequency of the eddy current probe oscillator. The measurement means isarranged to measure an amplitude of the demodulated frequency. Theamplitude comparison means is arranged to compare the measured amplitudewith one or more previously measured amplitudes, and/or a predefinedamplitude, and/or a predefined range of amplitudes. And in that thedetermining means is also arranged to determine the status by means ofthe result of the amplitude comparison means.

The aforementioned objects are also achieved according to the inventionby a vibration monitoring system arranged to monitor at least onerotating part by means of measurements from at least one eddy currentprobe. According to the invention the system comprises a distributedmodule locally to the at least one rotating part, the distributed modulecomprising a unit according to any of the above described embodiments.Suitably the distributed module is arranged to be located in a Zone 1,or equivalent, environment, and in that the at least one rotating partis located in a Zone 1, or equivalent, environment.

The aforementioned objects are further achieved according to theinvention by a vibration monitoring system arranged to monitor at leastone rotating part by means of measurements from at least one eddycurrent probe. According to the invention the system comprises adistributed unit locally to the at least one rotating part. Thedistributed unit is arranged to process the measurements digitally tothereby create an alarm signal. The distributed unit is further arrangedto digitally transfer the alarm signal by means of a at least doubleddigital data bus communication line to a machine shutdown controller.

With other systems it could be argued that the fact that tank impedancechanges become small when approaching infinity gap (far gap) as part ofthe non-linear response is a disadvantage. The ability to measure thesesmall impedance changes will increase the measurement range of theapplicable eddy current probe system. Therefore, the oscillator circuitshould be optimized for being sensitive to small impedance changes (highdV_(out)/dZ_(probe)) towards the maximum probe impedance.

Depending upon the intended monitoring application, the ECP systemdesign needs to adapt to numerous parameter changes. These variablesinclude, but are not limited to, displacement probe size, cable length,target material, and required output sensitivity.

Another principal limitation in the operational concept of other eddycurrent probe measurement systems is that it is not feasible to use longdistances with coaxial cable between probe and the final signalconditioning—i.e. a direct connection from the probe mounted in themachine to a centrally located monitoring system, perhaps severalhundred meters away. The present operational principles limit thisdistance to around 15 meters. Hence it is common practice to utilize a‘stand-alone’ driver to perform the required conditioning within theacceptable distance.

Other systems could be said to have some disadvantages. Analog designeddy current probe systems use an analog “driver” to perform thenecessary steps in-between the probe and the monitor. The variations inparameters such as probe size, cable length, cable parameters, etc. areaccommodated by ‘specially tuned’ derivatives of a standard analogcomponent range. This leads to many different components, which cannotbe easily interchanged, between different eddy current drive systems. Acommercial impact of such ‘tuning’ is that, in general, the probe, cableand driver must be all from the same manufacturer. For digital driversthere are known techniques that improve flexibility of a single eddycurrent probe system design (as opposed to the component variety of thefixed parameter based analog design). Once a linearization curve hasbeen established for an eddy current probe system, the system in generalremains static. Subsequently, the available signal processing power,used to establish the curve, remains unused but the component costremains. Also the cost of analog to digital conversion, digital signalprocessing and then digital to analog conversion (to allow interfacingwith standard, analog input based, monitoring and protection systems)would result in a commercially non-competitive product compared toproven analog designs. Other advantages of this invention will becomeapparent from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail for explanatory, andin no sense limiting, purposes, with reference to the following figures,in which

FIG. 1 illustrates an oscillator according to the invention,

FIG. 2 illustrates the oscillator non-linear transfer function,

FIG. 3 illustrates a block diagram of a driver according to theinvention,

FIG. 4 illustrates a block diagram of a digital driver according to theinvention,

FIG. 5 illustrates discrete and curve fit data of the probe impedance inrelation to position/distance between probe and target,

FIG. 6 illustrates the linearization process,

FIG. 7 illustrates a non-linearized probe curve response as measured bythe device,

FIG. 8 illustrates probe impedance (L, R) behavior versus distance/gap,

FIG. 9 illustrates modeling results of non-linear probe curve to a rangeof curves representing different capacitor settings,

FIG. 10 illustrates a plurality of linearization curves as a function ofdriver voltage,

FIG. 11 illustrates coefficient curve fitting examples,

FIG. 12 illustrates a flowchart of generation of coefficients accordingto the invention,

FIG. 13 illustrates a flowchart of calibration of eddy current channelwithout compensation according to the invention,

FIG. 14 illustrates a flowchart of calibration of eddy current channelwith compensation according to the invention,

FIG. 15 illustrates a flowchart of a compensation/linearization processaccording to the invention,

DETAILED DESCRIPTION

In order to clarify the method and device according to the invention,some examples of its functioning and use will now be described inconnection with FIGS. 1 to 15.

Instrumentation systems for the monitoring of vibration on criticalmachinery for the purposes of automatic shut-down and long-termequipment health, condition monitoring, requires vibration sensorsmounted on the machinery. Most common are radial displacement probessuch as eddy current probes, which need an analog oscillator/demodulatorunit, often known as a “driver” or “proximitor”. Other types of probesare “Seismic” sensors that measure surface vibration in eitheracceleration or displacement, and often do not require a driver. Adriver will comprise an oscillator used to excite the attached eddycurrent probe so that it can generate a changing magnetic field. Thismagnetic field, when in close range to a steel target material, willinduce time changing eddy currents in the surface of the target materialin relation to the close range. These eddy currents, in turn, willgenerate a magnetic field that will oppose the originating field andtherefore affecting the resulting impedance of the probe tip. The sizeof the induced eddy currents is dependent on the distance between theprobe tip and the steel target material. The probe impedance change istherefore a direct measurement of the distance between the probe tip andthe target material. This non-contact method can therefore be utilizedfor measurement of distance/gap (average DC component) and vibration oftarget machine axis (AC component).

The complex probe impedance is defined as:Z _(P) =R+jωL

Where the resistance R represents the wire resistance of the coil of theprobe and covers most of the probe losses as a result of magnetic fieldenergy absorption by the target material. The inductance L representsthe self-inductance of the coil and also has a relatively smallcontribution to the probe losses as a result of the magnetic fieldcoupling to the target material. The latter implies small oscillationfrequencies over the working gap range of the probe.

A special case is identified as the infinite gap or far gap condition.This represents the probe impedance NOT affected by magnetic fieldcoupling losses. This therefore represents the pure probe coilproperties consisting of coil wire resistance and self-inductance andthis is identified as Z₀.

The probe and the coaxial interface cable together form a parallel LCtank circuit, as the coaxial cable can be identified as a ‘distributed’capacitor. The probe/cable combination is usually described by a lossytransmission line (cable) loaded with the probe impedance (coil).

The output tank impedance (probe and cable) is an integral part of theoscillator circuit. The used oscillator type is a self-generatingoscillator. At startup of the system, a charge will be applied to thecapacitor of the tank circuit. The capacitor will discharge its energyto the inductor (coil). Due to the losses introduced by the voltage dropof the coil wire resistance the oscillation would dampen out. Howeverthe oscillator circuit will ensure that energy losses are compensatedfor and in addition ensure that oscillation amplitude will be keptconstant at a defined amplitude.

A very important concept is the fact that oscillation only occurs whenthe tank impedance is purely resistive. In this so-called resonance modethe tank output impedance is purely resistive and therefore theresulting voltage response is a direct measure of the probe positionrelative to the target. The resonance (oscillation) frequency ispredominantly defined by the probe inductance and cable capacitance.Consequently, when different probe types (inductances) or variations incable lengths are used this will affect the oscillation frequency. Itcan be noted, that the influence of the probe to, target position willalso slightly affect the resulting oscillation frequency.

The oscillator output peak to peak voltage level will be a measure ofthe distance between probe and target, and the low frequency amplitudechanges (envelope) will be a measure of the distance changes over timeand thus represent target vibrations. The demodulator output willeliminate high frequency components and establish a high precision peakto peak detector that will be used as basic input for further digitalprocessing according to the invention.

The relationship between the impedance voltage response and theprobe-target distance is not linear, and therefore calls for furtherprocessing to obtain a linear relationship for distance measurements(generally described using an output sensitivity of 200 mV/mil forcurrent analog systems). Furthermore, as can be derived from the above,variations in probe (R, L) and cable (R, C) can affect the resultingtank output impedance and therefore introduce errors, especially acrossvarious products, and thus require manual calibration to ensure a fixedlinear output result.

The present invention also includes a shift into the digital domain,which offers the required flexibility to allow for automaticcompensation and linearization options according to the invention withthe objective to eliminate any need for manual calibration andrestrictions within fixed hardware solutions for compensation andlinearization (as used in current full analog designs). This isdiscussed further below. Taking into account a digital design core forhardware control and signal post-processing (linearization of thenon-linear probe impedance response), the following system concept isused for the driver system according to the invention.

To accommodate for various cable lengths per probe type, resulting indifferent maximum impedance responses of the probe/cable assembly,driver oscillator operation is based upon using the longest cable lengthas a reference for a linearization process and use of externalcompensation capacitors and/or impedance matching networks to allow theuse of shorter cable lengths. The main purpose for compensation is toobtain equal impedance response of actual cable length opposed to thereference cable length for the infinite gap measurement response. Withthis in mind the driver oscillator can be optimized for a singleimpedance range that will suit multiple probe types and cable lengths,and thus take advantage of optimum dynamic input range. Furthermore, theoscillator design is optimized to allow for independent control for bothamplitude level and impedance measurement sensitivity. These parameterscan then be digitally controlled and used for optimizing design fordifferent probe types. The most important parameter of the oscillatoraccording to the invention is its non-linear gain behavior. Thefunctionality allows for more gain at higher probe system impedances.When the probe impedance changes per distance unit is getting smaller asthe probe moves further away from the target, this feature according tothe invention will provide more output gain. The oscillator according tothe invention will thus to some degree provide a linearization, thusextending the usable measurement distance range of the probe system.

Naturally, the oscillator circuit is restricted by the boundaryconditions for oscillation of feedback loop unity gain (1) and feedbackloop 0° phase shift opposed to output signal. These boundary conditionsneed to apply for the fully anticipated impedance range of probe/cableto maintain required excitation oscillation.

The block diagram according to FIG. 1 illustrates the optimizedoscillator design based upon using operational amplifiers 110, 116 orequivalents as basic building blocks. The oscillator design uses a dualfeedback loop, of which a first leg is designated as a voltage limiter112 to create a defined voltage limited input 106 which remains in phasewith the output signal 105. Included in the voltage limiter 112 is ahigh gain stage to ensure voltage limiting during early stages ofoscillation startup. Gain stages a 111 and b 114 will, in combinationwith output voltage divider R 101 and Z_(probe) 100, ensure that overallfeedback gain is unity and in-phase with the output signal 105 to meetoscillation boundary condition. In some implementations the gain stagesa 111, b 115, and c 113 may be implemented with resistors and the adder115 just being a common point for the resistors implementing gain stagesa 111, b 114 around the buffer op-amp 116.

Due to the non-linearity of the voltage limiter 112, output voltage U107 will also contain higher harmonics of the oscillation frequency.When oscillating, the probe/cable circuit will be in resonance andtherefore acting as a band filter for the fundamental oscillationfrequency. Therefore, only the fundamental harmonic signal will becoupled back to the input 106. A gain factor c 113 is optionallyintroduced to account for this fixed non-linearity effect. The followingequations can be derived for this oscillator circuit:

$\begin{matrix}{U = {{a \cdot U_{in}} + {b \cdot c \cdot U_{Limit}}}} & \lbrack 1\rbrack \\{U_{out} = {\frac{Z_{Probe}}{R + Z_{Probe}} \cdot U}} & \lbrack 2\rbrack\end{matrix}$U_(out)=U_(in)   [3]

From equations [1], [2], and [3] a relationship can be defined for theoutput voltage:

$\begin{matrix}{U_{out} = {\frac{b \cdot c}{\frac{R}{Z_{Probe}} + \left( {1 - a} \right)} \cdot {U_{Limit}.}}} & \lbrack 4\rbrack \\{U_{out} = {\frac{b \cdot c \cdot Z_{Probe}}{R + {\left( {1 - a} \right) \cdot Z_{Probe}}} \cdot U_{Limit}}} & \left\lbrack 4^{''} \right\rbrack\end{matrix}$

From [4′] it can be identified that U_(out) 105 is a second orderfunction of Z_(probe) 100 and therefore has a non-linear character. Asthe probe/cable impedance will have a maximum value (infinite gap), themaximum oscillation amplitude U_(Max) is defined as:

$\begin{matrix}{U_{Max} = {\frac{b \cdot c}{\frac{R}{Z_{Max}} + \left( {1 - a} \right)} \cdot U_{Limit}}} & \lbrack 5\rbrack\end{matrix}$

U_(Limit) is defined in such a way that for a small output voltage U thelimit voltage level remains constant for the full output response asresult of probe/cable impedance changes. From equation [5] one canderive a relationship for U_(Limit) and substitute this into equation[4] to obtain a relationship of U_(out) relative to U_(max).

$\begin{matrix}{U_{out} = {\frac{\frac{R}{Z_{Max}} + \left( {1 - a} \right)}{\frac{R}{Z_{Probe}} + \left( {1 - a} \right)} \cdot U_{\max}}} & \lbrack 6\rbrack\end{matrix}$

Using equation [6] and differentiating the output voltage function tothe probe impedance, a relationship can be determined that will resultin the highest output voltage change relative to impedance change:

$\begin{matrix}{\frac{\mathbb{d}U_{out}}{\mathbb{d}Z_{Probe}} = {\frac{\frac{R}{Z_{Max}} + \left( {1 - a} \right)}{\left\lbrack {\frac{R}{Z_{Probe}} + \left( {1 - a} \right)} \right\rbrack^{2}} \cdot U_{\max} \cdot \frac{R}{Z_{Probe}^{2}}}} & \lbrack 7\rbrack\end{matrix}$

Using the result of equation [7] the maximum voltage change is definedas follows using the fact that the changes close to the maximum probeimpedance (Z_(Max)) are mostly relevant in order to extend probe range:

$\begin{matrix}{{\frac{R}{Z_{Max}} + \left( {1 - a} \right)} = 0} & \lbrack 8\rbrack\end{matrix}$

From the result of equation [8] it can be concluded, that for a certainprobe type and probe cable length a certain Z_(Max) is known, thusleaving variables R and gain factor a to optimize the response.

Based upon maximum probe system impedance, output voltage and limitvoltage a fixed relationship will exist between parameters a, b and R.By means of selecting two parameters, the remaining parameter, basedupon pre-defined maximum settings, can be determined using equation [5],rewritten as equations [9], [10], and [11]:

$\begin{matrix}{{a\left( {b,R} \right)}:={\frac{R}{Z_{Max}} + 1 - {b \cdot c \cdot \frac{U_{Limit}}{U_{Max}}}}} & \lbrack 9\rbrack \\{{b\left( {a,R} \right)}:={\frac{U_{Max}}{c \cdot U_{Limt}} \cdot \left( {\frac{R}{Z_{Max}} + 1 - a} \right)^{25}}} & \lbrack 10\rbrack \\{{R\left( {a,b} \right)}:={Z_{Max} \cdot \left( {\frac{b \cdot c \cdot U_{Limit}}{U_{Max}} - 1 + a} \right)}} & \lbrack 11\rbrack\end{matrix}$

The behavior of the U(z) relationship, as defined in [1], depends on theparameter settings a, b and R. Within this respect it has to be notedthat for a defined maximum output voltage at maximum expected probesystem impedance a variety of options are possible to meet theserequirements. However, as can be derived from equation [1] the functionU(z) heavily depends on parameters a, b and R.

Parameters a and b also determines the non-linear behavior of theoscillator transfer function. If a<1 and b>1 then the transfer functionwill be a non-linear behavior where low impedance values are amplifiedmore than higher impedance values. If a=1 then the transfer functionwill be a linear behavior between output voltage and impedance. Finallyif a>1 and b<1 then the transfer function will be a non-linear behaviorwhere high impedance values are amplified more than lower impedancevalues.

FIG. 2 illustrates a situation where the oscillator shows a non-linearoutput response 288 with a=1.272, b=0.153, and R=604. Thus when theprobe impedance Z_(probe) 284 increases the output U_(out) 285 isamplified more. We can in this manner stretch small impedance changesper distance movement already by means of the oscillator.

With a fixed relationship of parameters a and b to optimize oscillatorresponse as a first step in linearizing the non-linear probe impedanceresponse, and with the fixed parameters U_(Limit) and calculationconversion factor c, this leaves only parameter R (combined with a knownZ_(Max) of the ECP system) to determine the maximum output voltage.

From equation [6] it can be concluded that the output voltage change,relative to defined maximum voltage, is purely dependent on theprobe/cable impedance change given the defined relationship of R and a(and b) of equation [8].

Based upon the above design the objective to have independent control ofoutput signal amplitude and optimization of impedance change response,the following control parameters apply:

-   -   1. Non-Linearity: combination of parameters a and b.    -   2. Amplitude gain: resistor value R (in combination with a fixed        gain factor a)

With reference to the above and equation [5] it can be concluded thatsome level of hardware optimization, for different conditions, can beobtained when R is matched with the actual impedance of the probe (Zp)that may be different from target due to changes in probe and cablecharacteristics. In general these changes will have a much higher effectwhen the cable length becomes longer and therefore will not be able tobe compensated in combination with the linearization function.

The driver according to the invention is thus a probe excitation basedupon using a ‘self-generating’ oscillator and not a design of having aprobe/cable system being tuned into resonance using a variable frequencysignal generator. The design advantages of the present invention aremany, such as a defined, mainly resistive probe/cable tank impedanceload. By using conventional op-amps, or equivalents, the tank impedanceload will not consist of active parasitic capacitive impedances. Thecircuit according to the invention is a voltage controlled loop. Byusing a highly accurate voltage limiter, using the output signal as aninput, and using highly accurate gain defining resistors, the amplitudevariations as a result of component issues, equal tank impedance gain,will have a high accuracy between different products. This will eitherpartially or completely eliminate the need for manual hardwarecalibration. The circuit is frequency independent. This will eliminatethe need for parallel compensation inductors, which may change responseover time. The circuit design according to the invention is a lowcurrent design to meet Intrinsic Safety approval for zone 1. Any mentionto a specific hazardous zone classification such as zone 1 is accordingto zone 1 or according to an equivalent classification such as class 1division 1. There is no manual range calibration requirement, and thereis a high level of module interchangeability—‘one module supports allprobes’ concept. However, the resistor value R can be tuned tocompensate for resistive loading of probe/cable tank impedance circuitby the oscillator circuit.

FIG. 3 illustrates a driver 352 comprising an oscillator circuit 390according to the invention. The circuit 390 according to the inventionforms one part of the complete oscillator circuit 341 which alsocomprises a probe 302 and a cable 303. Following the output of theoscillator 305 is a high-speed peak-peak detector 342 to convert thedynamic high frequency output signal into a DC voltage output signal 308representing the gap between probe and target and superimposed the shaftvibration signal (low frequency up to 10 kHz). A driver 352 commonlycomprises the demodulator, low pass filter and peak to peak detector 342functions and the oscillator 341. This driver 352 comprising theoscillator circuit and demodulator and peak to peak detector 342 can beused in a stand alone configuration where the output signal 308 istransferred to a central processing place, or it can be integrated withdistributed digital processing means according to the invention asdescribed later.

A driver according to the invention as described above can beimplemented in an instrumentation room based system. There is a need tomonitor vibration on critical machinery in high ignition riskenvironments, such as they exist in the energy sector, for example theoil & gas business, for the purposes of automatic shut-down andlong-term equipment health, condition, monitoring. In high ignition riskindustries such as the oil & gas business the control room is designateda safe area with respect to flammable gases.

A driver according to the invention as described above can also beimplemented in a machine based system. In many businesses/industries,such as the oil & gas business, a machine is designated a ‘hazardousarea’ with respect to flammable gases—and divided into ‘zones’ ofignition. Any electrical device located in a hazardous area must bedesigned such that—in the event of a component failure—an ignitionsource (e.g. spark) is either impossible or contained, and cannot reachthe flammable gas. The concept of an intrinsically safe monitoring andprotection system with a distributed nature according to the inventionresolves many disadvantages. To enable the invention, the eddy currentdriver according to the invention described above, is integrated withdigital signal processing to thereby form a complete unit integratingvibration monitoring and part of the condition monitoring, thatcommunicates over digital signal lines according to the invention to thedistributed control system and a central condition monitoring part. Thisunit can comprise a plurality of drivers and inputs for sensors notrequiring drivers.

Analog systems, as described above, mainly comprise three separatecomponents to form an eddy current system—a displacement probe, a cableand an oscillator/demodulator, usually known as a driver or proximitor.The probe driver performs probe activation/excitation and in many casesalso analog signal linearization and some signal conditioning, to beready for input to an external monitoring and protection system device.If the driver comprises analog circuitry for linearization and signalcondition for a variety of eddy current displacement probe types, thedriver becomes very complex.

Depending upon the intended monitoring application, the eddy currentprobe system design systems needs to adapt to numerous parameterchanges. These variables include, but are not limited to, displacementprobe size, cable length, target material, and required outputsensitivity. Another principal limitation in the operational concept ofthe eddy current probe measurement is that it is not feasible to uselong distances with coaxial cable between probe and the final signalconditioning—i.e. a direct connection from the probe mounted in themachine to a centrally located monitoring system, perhaps severalhundred meters away. The present operational principles limit thisdistance to around 15 meters. Hence the use of a stand-alone driver toperform the required conditioning within an acceptable distance from theprobe.

A digital driver will improve the flexibility of a single eddy currentprobe system design, as opposed to the component variety of the fixedparameter based analog design. However, a digital driver—as astand-alone product—will in general remain static once a linearizationcurve has been established for an eddy current probe system.Subsequently, the available signal processing power, used to establishthe curve, remains unused but the component cost remains. The cost ofanalog to digital conversion, digital signal processing and then digitalto analog conversion to allow interfacing with standard, analog inputbased, monitoring and protection systems, would result in a commerciallynon-competitive product compared to proven analog designs.

With the introduction of distributed condition and protection monitorslocal to the machine according to the invention, the final signalconditioning can be performed within the distance limitation of the eddycurrent probe (ECP) system, and the need for individual sensor wiring toremote monitoring and protection systems is eliminated. This invention,based upon the predominantly digital signal processing character of adistributed monitoring and protection module, therefore includes thedriver signal conditioning hardware into the product, and utilizes thedigital signal processing power for the required linearization in a morecost effective manner, as the main function of digital signal processingis continuous monitoring and protection function of connected sensors.An additional effect of the inclusion of the driver hardware into thatpart of the invention is the full elimination of any required digital toanalog conversion and copper wire interface between driver and monitor,thus optimizing cost, effectiveness and overall system quality andreliability. Adding further to reliability, this system also monitorsthe presence of the RF probe excitation signal in addition to the commonDC output level probe OK monitoring of the driver output. Therefore, theinvention provides a higher diagnostic coverage, in addition to regularDC based OK monitoring, for probe system failures. Configurationsoftware will allow for in-situ calibration and linearization ofdirectly attached displacement probe types and various cable lengths(tailored to the needs of different applications).

The system according to the invention allows an overall system costreduction by excluding the need for separate analog driver system. Thecertification that permits location and operation of theinvention—including integrated eddy current probe driver signalconditioning—in a Zone 1 hazardous area on the machine base itself isoptimized by hardware integration for distributed use. Increase thereliability and reduce maintenance issues by support for flexible, butrestricted, displacement probe cable lengths using a single driverdevice. Increase reliability and reduce maintenance issues by supportfor different make and type of displacement probes using a single driverdevice. There is also according to the invention a software-drivenin-situ calibration and linearization option.

FIG. 4 illustrates the major parts of the distributed part of adistributed vibration and condition monitoring system according to theinvention. The distributed part comprises one or more eddy current probeinterfaces to thereby make up the oscillator 441 part which in turndelivers a high frequency output 405. Following the output 405 of theoscillator 441 is a high-speed peak-peak detector 442 to convert thedynamic high frequency output signal into an output signal 408comprising a DC voltage representing the gap between probe and targetand a superimposed low frequency signal up to about 10 kHz representingthe shaft vibration signal. This demodulated and peak to peak detectedoutput signal 408 is thereafter analog to digital converted in an ADconverter 492 before the then digital signal is brought into the digitalsignal processing part 494. After processing and data reductionaccording to the invention, explained in detail below, data istransmitted via an I/O interface 496 to a distributed control system forshut-down control.

One approach to perform probe function monitoring for eddy current probesystems is to monitor the DC gap voltage. However this method ofdetection depends not only on the probe/cable but also upon properoperation of a substantial amount of electronic circuitry such as theoscillator, peak to peak detector. This method cannot therefore be 100%conclusive in identifying actual malfunction of the probe and/or cableinterface. In addition this approach is particularly ineffective inthrust monitoring applications where the DC gap measurement is both themeasurement parameter and the probe control measure. Hence the system isunable to distinguish between a simple out of range and actual componentfailure. Correct function detection according to the invention based onoscillation frequency provides such a distinction and if employedtogether with DC voltage monitoring allows for the detection andidentification of both out of range and failure. In this situation theoscillation frequency can be monitored and in the event this is outsidean expected range a probe or (oscillator) circuit failure can bedetermined at an earlier stage. The high frequency signal 405 istherefore connected to the digital signal processing part 494 forfrequency measurement and thereafter range checking. This method willtherefore positively contribute to a more reliable and redundant methodfor probe function monitoring.

Furthermore, when monitoring the actual oscillation frequency, one cancompare the response with predefined specific probe system data.Different probe types may operate at various oscillation frequencies.When a frequency deviates from a predefined range according to aconfiguration, this will indicate a mismatch between the configurationand the connected probe type and can be used to prevent possibleshutdown as result of human error, i.e. use of a wrong probe type or awrong configuration. Similar, for a correct configuration, a warning canbe issued when the frequency and/or the amplitude response moves awayfrom a nominal operating value, indicating that the impedance ischanging as a result of physical changes to the system, i.e. change ofprobe coil characteristics as a result of environmental conditions. Inaddition to the frequency monitoring—during system setup—the infinitivegap response can be compared against expected values. In case theoscillation frequency is as expected but a significant change isidentified in amplitude response this will identify a cable withdifferent cable characteristics and will therefore require newcalibration information, i.e. a replacement of an extension cable with adifferent characteristics than the previous version.

Based upon the ability of frequency and amplitude monitoring advancedimpedance correction techniques, in principle similar to the nextdescribed linearization techniques according to the invention, can beused to compensate for non probe-target distance related systemcharacteristic changes, such as resistive probe cable losses.

Due to non-linear behavior of the probe position response, output signalprocessing is required to obtain a linear response. The probe/cableimpedance is directly related to the probe type, probe coil parameters,the probe position, the cable parameters, the cable length, oscillationfrequency and the target material. Based upon experience there is noneed for accommodating automatic methods to respond to different targetmaterials etc. Linearization is based upon known target materials, probetypes and cable characteristics and will accommodate for a practicalrange of cable lengths.

The used linearization method is predominantly based upon compensatingfor capacitance influences as introduced by probe cable, inputcapacitance differences between modules and input channels. Main inputfor this linearization process is the difference in infinite gap valueresponse. This method allows for elimination of the need for individualcomponent trimming (probe cable, extension cable and driver) andtherefore reduces field calibration and maintains a high level ofinter-product replacement ability, besides manufacturing cost savings.

However, besides capacitance changes, there will also be probedifferences that will manifest themselves in changes in coil tipinductance values. These variances can be caused by deviations in thewinding process (wire diameter, tension, uniformity, number of turns,etc.). The inductor variances will manifest themselves predominantly ina change of oscillation frequency, but of course also have influence onthe infinite gap response in similar way as change of capacitance in theprobe oscillation tank circuit. When probe systems are linearized basedupon the capacitance change based linearization concept, this is basedupon equal non-linear behavior of the probe tip.

In principle we have two methods for compensating for probe tipvariations. The first method is called Pre-Linearization Compensation,this method, basically similar to the actual linearization method, willupdate each measurement sample to the probe tip coil characteristics asused for linearization library generation. An advantage of this methodis that the compensation is applied on the full measurement range andprovides best results. A disadvantage is that this involves processingtime on a per sample basis using a polynomial high order algorithm. Thesecond method is called Post-Linearization Compensation, this method isbased upon the behavior that the linearization process results inrelatively small changes of output sensitivity that will show largerdeviations at higher gaps. This option therefore allows for optimizingthe internal system offset and gain factors during calibration processand therefore have the major advantage that it will involve no furtherprocessing time (and power).

These methods allow for elimination of the need for individual componenttrimming, probe cable, extension cable and driver, and therefore reducefield calibration and maintain a high level of inter-product replacementability. In the event that system characteristics have changedsignificantly, the system allows for system recalibration based uponfactory originated linearization calibration data. There is no need forin-field system calibration. The main field requirement is to obtain theinfinite gap response of a probe in question prior to installation. Theunit stores probe identification information in case these tasks areexecuted prior to final system installation. Measured infinite gapresponse is used as a basis to determine the actual sets of coefficientsrequired for executing the compensation of the impedance measurementvoltage and linearization of the result thereof. However, there are nogiven restrictions to execute a specific field calibration to obtainlinearization coefficients for a specific eddy current probe system,i.e. target material, probe type and cable length.

FIG. 5 illustrates the non-linear behavior of the impedance 584 inrelation to the distance 582 to the target. This is shown both for thediscrete data (squares) and a curve fit data (line) 586. Input for proberesponse linearization is the oscillator driver response, peak to peakmeasurement result of oscillator output voltage. Based upon the desiredoutput sensitivity the linearization curve can be defined using thefollowing formula:

$\begin{matrix}{{{Lin}({Pos})} = \frac{{Pos} \cdot {Sensitivity}}{U_{out}({Pos})}} & \lbrack 12\rbrack\end{matrix}$

Using this linearization function [12] and the function describing thenon-linearized output of the oscillator the linearized output responseis defined as:Output(Pos)=U _(out)(Pos)·Lin(Pos)  [13]

FIG. 6 illustrates the oscillator response voltage 670, 685 as functionof gap 682 (distance between probe and target), the requiredlinearization multiplier function 672, 685 as result of desiredsensitivity of 200 mV/mil and the resulted linearized output 674, 686.The objective of the linearization process is to describe thelinearization function in an n^(th) order polynomial function to allowfor on-the-fly linearization of each driver output sample. A polynomialfunction has been selected as this has a high computational calculationefficiency in combination with hardware multiplier circuitry and thuscan be executed fast and reduce overall power consumption. However, forevery difference in cable length, probe type, etc., the linearizationfunction will be different. To overcome this issue, multipleexperimental curves per probe type are required at reference cablelength with small cable capacitance deviations that simulate small cableand probe characteristic deviations and length errors, in order tocharacterize the range of linearization functions.

As the above principle requires a lot of practical effort a methodaccording to the invention has been introduced to predict the behaviorof capacitance variations (cable length differences, inter channel orinter module input differences between ECP inputs) based upon a singlereference curve. This mathematical method will calculate the response ofmultiple curves used as input to create the so-called familylinearization libraries used to calibrate the ECP channels.

The intention of this Linearization Prediction model is to make atheoretical prediction of a certain capacitance change range based upona standard probe single curve measurement. One channel is used as areference channel as this is available on all models and it's deviationcompared to other ECP input channels is somewhere in the middle. Thisconcept requires the actual probe behavior to be small as inductance andresistance variances between probes will result in a differentnon-linear behavior which basically will not be corrected for using thecapacitance variance compensation.

Practical tests have shown that for a group of probes the standarddeviation between the quotient of Infinite Gap value and Oscillationfrequency provides a good measure for probe equality. Therefore it willbe a main objective for probe manufacturing to ensure that theseproperties will be within specs that result from the actual familylinearization library results.

First, as can be seen in FIG. 7, a probe response 783 is measured 790 inrelation to gap 782. Based upon this unity gain non-linearized curve theECP input impedance is calculated based upon known transfer functions ofthe ECP digital and hardware circuitry. With a known cable compensationcapacitance and ECP hardware input impedance, calculate the probe systemoutput impedance. Based upon using lossy transmission line theoryapplied to probe cable and extension cable length sections, known cablecharacteristics and inclusion of connector insertion losses, the probeimpedance is calculated based upon fact that probe system outputimpedance is a real impedance when in resonance. The result, as can beseen in FIG. 8, is a measure of the probe tip coil inductance L 884, 891and loss resistance R 885, 891 for every evaluated gap 882.

Then according to the invention, with the known probe impedance(including module load), the expected response with differences incompensation capacitors (simulating variances in cable length, cablespec variations and differences between input channels) is calculated byreversing the lossy transmission line calculation from probe tip tooscillator input. Due to the change in capacitance a change in frequencyis expected and this will affect the resulting resistive impedance whenin oscillation. As can be seen In FIG. 9, based upon the newlycalculated probe response curves, the known hardware transfer functionscan be used again to calculate multiple non-linear gap 982 response 995curves 993—including associated calculated infinite gap responses—to beused as input for generating the probe family linearization library.

Each probe response (discrete samples) will be translated into therelated linearization function and fitted to an n^(th) order polynomialfunction for each of the evaluated k number of input curves from FIG. 9.Lin_(k)(x)=a(1)_(k) +a(2)_(k) x+a(3)_(k) x ² +a(4)_(k) x ³ + . . .+a(n+1)_(k) x ^(n)  [14]

As illustrated in FIG. 10, for further calculations the linearizationmultiplier 1084 curves 1073 as a function of the oscillator outputvoltage 1085 response is used, and described by the n^(th) orderpolynomials and related sets of coefficients. The curves 1073 comprisethe measured curve and the calculated curves with varying C from themeasured curve conditions, as described above.

With the objective of defining a target linearization curve of anunknown probe/cable system within the range of calculated referencecurves, the required input of the unknown system will be an infinite gapmeasurement response. To use the infinite gap measurement response valueto define a set of coefficients which best describe the targetlinearization curve, each corresponding coefficient of the calculatedreference linearization curves is curve fitted to a corresponding m^(th)order polynomial using the applicable infinite gap response asreference. This will result in a function that will provide the curvefit result of an individual linearization curve fit coefficient basedupon use of the known difference between library infinite gap andmeasured infinite gap response. As the calculated linearization curvesare described with an n^(th) order polynomial this will result in n+1coefficients. Similar, each of these coefficients will be described byan m^(th) polynomial function with the infinite gap measurement responseas input. Consequently the (n+1)*(m+1) coefficients will therefore formthe linearization data set. From the above the following functions canbe derived describing the coefficients of the desired linearizationcurve of the unknown probe system with y=Stored library InfiniteGap−Measured Infinite Gap response:Coeff_(a(1))(y)=b ₁(1)+b ₁(2)y+b ₁(3)y ² + . . . +b ₁(m+1)y ^(m)Coeff_(a(2))(y)=b ₂(1)+b ₂(2)y+b ₂(3)y ² + . . . +b ₂(m+1)y ^(m). . .Coeff_(a(n+1))(y)=b _(n+1)(1)+b _(n+1)(2)y+b _(n+1)(3)y ² + . . . +b_(n+1)(m+1)y ^(m)  [15]

Therefore, with calculated reference linearization curves, and ameasured infinite gap response with an unknown linearization curvewithin the boundaries of the reference curves, one can calculate thecoefficients describing the desired unknown linearization curve.

The following function, using the driver sample voltage as input (x),can now be used to obtain the linearized response for each acquired datasample:U _(out)(x, y)=Coeff_(a(1))(y)+Coeff_(a(2))(y)·x+Coeff_(a(3))(y)·x ²+ .. . +Coeff_(a(n+1))(y)·x ^(n)  [16]

The number of reference curves (k) and the order of the used polynomials(n and m) will determine the accuracy and the working range (spreadbetween reference linearization curves) of the output results.

FIGS. 11A and 11B show an example of the curve fitting of the first andsecond coefficients of a linearization curve function using a 4th orderpolynomial and 5 reference curves. FIG. 11A shows the values 1182 of thefirst coefficients 1192 of the 5 reference curves in relation to theircorresponding infinite gap value 1180 being curve fitted 1172. FIG. 11Bshows the values 1184 of the second coefficients 1194 of the 5 referencecurves in relation to their corresponding infinite gap value 1180 beingcurve fitted 1174. To then get the desired first and second coefficientvalues one enters the measured infinite gap value into each of thesegraphs and extracts the coefficients from each of the fitted curves1172, 1174. As clearly identified in this example, the number ofreference curves will improve the accuracy on the coefficients.Therefore the use of the referenced mathematic model is preferred abovea practical method as in principle an unlimited number of coefficientscould be used. In principle when using small intervals between thecurves, a single set of coefficients could be generated to describe thecomplete range of cable lengths for a specific probe type.

Linearization and prediction method are based upon capacitive changesand therefore accommodate for cable length variations and capacitivedifference between the channels and different modules. Probe calibrationusing this family range will then compensate for small capacitivedifferences. However, this method is based upon an equal non-linearityof the probe system used. In case of variations in the probe coil thiswill result in a different non-linear behavior. The earlier mentionedand most computationally efficient post-linearization compensationmethod will be described in this section.

The base principle for the post-linearization compensation method isagain prediction based upon a theoretical model of non-linearityevaluation as a result of probe tip coil variances compared to theresponse of a probe reference system. As where the infinite gap value ismain reference for the linearization process the compensation method isbased upon the quotient of the infinite gap and oscillation frequency ofprobe system divided by the quotient of the infinite gap and oscillationfrequency of the reference probe system, further identified as theinfinite gap/oscillation quotient.

First probe manufacturing test data is used to get a curve response fora range of probe coil variances, or possibly a simulation model is usedfor this purpose. Secondly for Non-linearity difference evaluation forevery curve calculate actual linearization coefficients and calculatedlinearization coefficients based upon reference probe system. Create acompensation library function based upon an array of individualcoefficients (4th order polynomial function; a0, a1, etc.) and thecorresponding reference of infinite gap/oscillation quotient. Then forevery curve calculate the linearization output response based upon thereference probe system linearization library and calculate first ordercompensation function (offset and gain). Create a compensation libraryfunction based upon array of compensation functions and thecorresponding reference of infinite gap/oscillation quotient.

With the resulting compensation function the calculated linearizationcoefficients can be adapted accordingly during the calibration processto optimize the non-linearity differences between probe systems comparedto reference system. Additional advantages of this method usingmanufacturing data is that it provides direct means of probemanufacturing quality control as probes need to fit into thecompensation range as defined by this method. Various stages in theprobe manufacturing process can then be tested whether the response iswithin pre-defined acceptance regions.

With reference to the overall process of the presented compensation andlinearization process, three major steps can be identified, generationof coefficients, calibration of eddy current channel, and acompensation/linearization process. Generation of coefficients, basedupon a number of reference curves under defined conditions, generate therequired sets of coefficients that will characterize the compensationand linearization function for a specific probe type and range ofsupported cable lengths. These are factory-originated data sets thatform “library data” for a specific probe type. These data sets arestored and can subsequently be downloaded to monitoring modules, asrequired. FIG. 12 shows a flowchart indicating the steps of generationof coefficients according to the invention. In a first step 1210according to the invention, there is reference curve data acquisitionincluding infinite gap, voltage vs. position. After the first step 1210,in a second step 1220 linearization curves are created, factor vs.position. After the second step 1220, in a third step 1230 nth orderpolynomial curve fit is applied, n+1 coefficients. After the third step1230, in a fourth step 1240 it is tested if all K samples are done, ifnot then the procedure goes back to the first step 1210. If all Ksamples are done, then the procedure continues with a fifth step 1250.After the fourth step 1240 if all K samples are done, then in a fifthstep 1250 an mth order polynomial curve fit is applied for eachindividual coefficients of k samples, m+1 coefficients. After the fifthstep 1250, in a sixth step 1260 it is tested if all n+1 coefficients aredone, if no, then the procedure goes back to the fifth step 1250.Otherwise if all coefficients are done, then the procedure continueswith a seventh step 1270. After the sixth step, if all coefficients aredone, then in the seventh step n+1 sets of m+1 coefficients are stored.

Calibration of eddy current probe channel, based upon probe type andcable length, load the appropriate sets of coefficients, install probesystem complete with compensating capacitor and measure infinitive gapresponse and calculate/store the compensation and linearization functioncoefficient for the applicable eddy current probe channel. This is amodule function, usually carried out once, at eddy current probe systeminstallation. FIG. 13 shows a flowchart indicating the steps ofcalibration of eddy current channel without compensation according tothe invention. In a first step 1310 according to the invention, n+1 setsof m+1 coefficients for probe type and cable length are loaded. Afterthe first step 1310, in a second step 1320 the target probe system isinstalled. After the second step 1320, in a third step 1330 infinitivegap system response is measured and stored. After the third step 1330,in a fourth step 1340 n+1 sets of coefficients are calculated. After thefourth step 1340 in a fifth step 1350 n+1 linearization functioncoefficients for the ECP channel are stored.

FIG. 14 shows a flowchart indicating the steps in the procedure ofcalibration of eddy current channel with compensation according to theinvention. In a first step 1410 according to the invention, n+1 sets ofm+1 coefficients for probe type and cable length are loaded. After thefirst step 1410 in a second step 1420 compensation function coefficientsare loaded. After the second step in a third step 1430 the target probesystem is installed. After the third step 1430 in a fourth step 1440infinitive gap system response and oscillation frequency system responseis measured and stored. After the fourth step 1440 in a fifth step 1450n+1 sets of linearization coefficients are calculated. After the fifthstep 1450 in a sixth step 1460 compensation coefficients are calculated.After the sixth 1460 step in a seventh step 1470 the linearizationcoefficients are corrected with compensation coefficients. Finally afterthe seventh step 1470 in an eighth step 1480 n+1 linearization functioncoefficients are stored for the ECP channel.

For every eddy current probe channel the (compensated) linearizationcoefficients are loaded and driver sample data acquisition is startedand compensation is applied to each individual sample and thelinearization function is applied to the result thereof. This is amodule function where linearization is carried out “on-the-fly” as partof the sampling process and the linearized samples are then furtherprocessed by digital filtering and signal assessment techniques. FIG. 15illustrates a flowchart of the Compensation/Linearization processaccording to the invention. In a first step 1510 according to theinvention, n+1 (compensated) linearization coefficients are loaded forthe ECP channel. After the first step 1510 or a fourth step 1540 in asecond step 1520 an ECP measurement sample is taken/retrieved. After thesecond step 1520 in a third step 1530 a compensated measurement sampleis applied to the linearization function. After the third step 1530 inthe fourth step 1540 the linearized sample value is stored and theprocedure then returns to the second step 1520.

The invention is not restricted to the above-described embodiments, butmay be varied within the scope of the following claims.

FIG. 1 - illustrates an oscillator according to the invention, 100Probe/Probe impedance Z, 101 Resistor R, 105 Uout, 106 Uin, 107 U, 110×1 buffer, 111 times a multiplier/buffer, 112 Ulimiter, 113 optionaltimes c multiplier, resistor 114 times b multiplier/buffer, resistor 115adder, adding point 116 ×1 buffer, FIG. 2 - 284 probe impedance 285driver output 288 function between probe impedance and driver outputFIG. 3 - illustrates a block diagram of a driver according to theinvention, 302 Probe, 303 Cable, 305 High frequency output signal fromoscillator part, 308 Amplitude output signal from demodulator/peak topeak detector, 341 “Oscillator” part of driver, including probe andcable as these are a part of the complete oscillator, 342 Demodulatorand peak of peak detector, 352 Driver according to the invention, 390Excitation part of oscillator according to invention FIG. 4 -illustrates a block diagram of a digital driver according to theinvention, 405 High frequency output signal from oscillator part, 408Amplitude output signal from demodulator/peak to peak detector, 441“Oscillator” part of driver, including probe and cable as these are apart of the complete oscillator, 442 Demodulator and peak to peakdetector, 492 Analog to digital converter, 494 Digital signalprocessing, 496 Input/Output interface. FIG. 5 - illustrates discreteand curve fit data of the probe impedance in relation toposition/distance between probe and target, 582 Position/distancebetween target and probe, 584 Probe impedance in Ohms, 586 Curves. FIG.6 - illustrates the linearization process, 670 Probe driver outputcurve, 672 Linearization multiplier factor curve, 674 Linearized outputcurve (line), 682 Position/distance between probe and target material inmils, 685 Both the driver output in Volts and the Linearizationmultiplication factor, 686 Linearized driver output in Volts, FIG. 7 -illustrates a non-linearized probe curve response as measured by thedevice, 782 Gap between probe and target material 783 Response 790Measured probe response curve FIG. 8 - illustrates probe impedance (L,R) behavior versus distance/gap, 882 Gap between probe and targetmaterial 884 Inductance (H) 885 Resistance (Ohm) 891 probe coilinductance curve vs. gap 892 probe coil resistance curve vs. gap FIG.9 - illustrates modeling results of non-linear probe curve to a range ofcurves representing different capacitor settings, 982 Gap between probeand target material 993 Calculated probe response curves with varying C995 Response FIG. 10 - illustrates a plurality of linearization curvesas a function of driver voltage, 1073 A plurality of differentlinearization curves for different probes/cables . . . , 1084Linearization multiplier factor. 1085 Driver output voltage, FIGS. 11A &11B - illustrates coefficient curve fitting examples, 1172 Fitted curveto discrete Coeff(0), 1174 Fitted curve to discrete Coeff(1), 1180Infinite gap voltage, 1182 Coefficient (0) scale, 1184 Coefficient (1)scale, 1192 Discrete points Coeff(0), 1194 Discrete points Coeff(1),FIG. 12 - illustrates a flowchart of generation of coefficientsaccording to the invention, 1210 a first step of according to theinvention, Reference curve data acquisition including infinite gap,voltage vs position, 1220 after the first step: a second step of, Createreference linearization curves, factor vs position 1230 after the secondstep: a third step of apply nth order polynomial curve fit, n + 1coefficients, 1240 Fourth step, All K samples done, if no back to thefirst step, if yes then the fifth step, 1250 Fifth step, Apply mth orderpolynomial curve fit for each individual coefficients of k samples, m +1 coefficients, 1260 Sixth step, all n + 1 coefficients?, if no, thenback top the fifth step, if yes then to the seventh step, 1270 Seventhstep, store n + 1 sets of m + 1 coefficients. FIG. 13 - illustrates aflowchart of calibration of eddy current channel without compensationaccording to the invention, 1310 a first step of according to theinvention, load n + 1 sets of m + 1 coefficients for probe type andcable length 1320 after the first step: a second step of, install thetarget probe system 1330 after the second step: a third step of measureand store infinitive gap system response 1340 fourth step, calculate n +1 sets of coefficients, 1350 fifth step, store n + 1 linearizationfunction coefficients for ECP channel. FIG. 14 - illustrates a flowchartof calibration of eddy current channel with compensation according tothe invention, 1410 a first step of according to the invention, load n +1 sets of m + 1 coefficients for probe type and cable length 1420 afterthe first step, a second step of loading compensation functioncoefficients 1430 after the second step: a third step of, install thetarget probe system 1440 after the third step: a fourth step of measureand store infinitive gap system response and oscillation frequencysystem response 1450 after the fourth step, a fifth step, calculate n +1 sets of linearization coefficients, 1460 after the fifth step, a sixthstep to calculate compensation coefficients 1470 after the sixth step, aseventh step to correct linearization coefficients with compensationcoefficients 1480 after the seventh step, an eighth step, store n + 1linearization function coefficients for ECP channel. FIG. 15 -illustrates a flowchart of a compensation/linearization processaccording to the invention, 1510 a first step of according to theinvention, load n + 1 (compensated) linearization coefficients for ECPchannel 1520 after the first step or fourth step: a second step to getECP measurement sample, 1530 after the second step, a third step step,apply compensated measurement sample to linearization function, 1520after the third step, a fourth step, store linearized sample value andreturn to second step.

1. A method of determining a status of an eddy current probe/cablesystem attached to an eddy current probe oscillator, wherein the statusrefers to a system type of the eddy current probe/cable system and/or acorrect functioning of the eddy current probe/cable system, comprisingthe step of: determining the status based on an oscillation frequency ofthe eddy current probe oscillator, and wherein the step of determiningstatus based on oscillation frequency is defined by probe inductance andcable capacitance.
 2. The method according to claim 1, furthercomprising the steps of: measuring a frequency of the eddy current probeoscillator, comparing the measured frequency with one or more previouslymeasured frequencies, and/or a predefined frequency, and/or a predefinedrange of frequencies, determining the status by means of the result ofthe frequency comparison.
 3. The method according to claim 1, furthercomprising the steps of: demodulating the frequency of the eddy currentprobe oscillator, measuring an amplitude of the demodulated frequency,comparing the measured amplitude with one or more previously measuredamplitudes, and/or a predefined amplitude, and/or a predefined range ofamplitudes, and wherein the step of determining the status alsocomprises determining the status by means of the result of the amplitudecomparison.
 4. A unit arranged to determine a status of an eddy currentprobe/cable system attached to an eddy current probe oscillator, whereinthe status refers to a system type of the eddy current probe/cablesystem and/or a correct functioning of the eddy current probe/cablesystem, the unit being configured to determine the status based on anoscillation frequency of the eddy current probe oscillator and whereinthe status determined based on oscillation frequency is defined by probeinductance and cable capacitance.
 5. The unit according to claim 4wherein the unit comprises: measurement means arranged to measure afrequency of the eddy current probe oscillator, frequency comparisonmeans arranged to compare the measured frequency with a one or morepreviously measured frequencies, and/or a predefined frequency, and/or apredefined range of frequencies, and determining means arranged todetermine the status by means of the result of the frequency comparisonmeans.
 6. The unit according to claim 5, wherein the unit furthercomprises: demodulation means arranged to demodulate the frequency ofthe eddy current probe oscillator, measurement means arranged to measurean amplitude of the demodulated frequency, and amplitude comparisonmeans arranged to compare the measured amplitude with one or morepreviously measured amplitudes, and/or a predefined amplitude, and/or apredefined range of amplitudes, and wherein the determining means isalso arranged to determine the status by means of the result of theamplitude comparison means.
 7. A vibration monitoring system arranged tomonitor at least one rotating part by means of measurements from atleast one eddy current probe, comprising a distributed module locally tothe at least one rotating part, the distributed module including a unitconfigured to determine a status based on a frequency of the eddycurrent probe, the status including at least one of a system type of theeddy current probe and correct functioning of the eddy current probe,and wherein the status is determined based on an oscillation frequencyof the eddy current probe oscillator, and wherein the status determinedbased on oscillation frequency is defined by probe inductance and cablecapacitance.
 8. The vibration monitoring system according to claim 7,wherein the distributed module is arranged to be located in a Zone 1environment, and in that the at least one rotating part is located in aZone 1 environment.